Exposing Zone Sleeps

[Sean Bruno]

Find attached a patch against -CURRENT.

This update exposes a counter that indicates the number of times that we
sleep when attempting to allocate a slab from the keg.  In other words,
the number of times we BLOCK and wait, which is bad.

This allows differentiation between times when we failed to allocate and
it was ok and times where we were forced to sleep.  The current FAIL
counter does not make this distinction.

Exposes this information via uma_zone_t->uz_sleeps.

Add a new sysctl to retrieve this information.
Enhance vmstat -z to retrieve this information.

We've found this *extremely* useful here at Yahoo in the past and would
like to commit this if it is acceptable.

Tested on 32bit and 64bit architectures on 6/7/CURRENT.

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